<template>
  <div id="app">
    <div class="box">
      <div class="left"></div>
      <div class="mid"></div>
      <div class="right"></div>
    </div>
  </div>
</template>
<style>
#app .box {
  display: flex;
  width: 100%;
  height: 100vh;
}
#app .box .left {
  flex: 1;
  background:red;
}
#app .box .mid {
  flex: 1;
  background-color: #ddd;
}
#app .box .right {
  flex: 1;
  background-color: aqua;
}

</style>
